The traditional implantable cardiac pacemaker includes a pulse generator device to which one or more flexible elongate lead wires are coupled. The device is typically implanted in a subcutaneous pocket, remote from the heart, and each of the one or more lead wires extends therefrom to a corresponding electrode, coupled thereto and positioned at a pacing site, either endocardial or epicardial. Mechanical and/or MRI compatibility issues, which are sometimes associated with elongate lead wires and well known to those skilled in the art, have motivated the development of implantable cardiac pacing devices that are wholly contained within a relatively compact package, the entirety of which is configured for implant in close proximity to the pacing site. FIG. 1 is a schematic diagram that shows potential cardiac implant sites for such a device, for example, within an appendage 102 of a right atrium RA, within a coronary vein CV (via a coronary sinus ostium CSOS), or in proximity to an apex 103 of a right ventricle RV. An implanting physician may employ a standard guiding catheter (not shown) to deliver a relatively compact medical device to any one of the three exemplary sites, for example, according to methods known in the art of interventional cardiology, by maneuvering the catheter, with the device loaded therein, up through the inferior vena cava IVC and into the right atrium RA. However, there is still a need for new interventional medical systems, assemblies and methods that increase the ease and efficiency of delivering such an implantable medical device to an implant site.